This invention relates to spatial light modulators used for video display systems, and specifically to methods and apparatus for generating grayscale and full-color video images on such display systems.
The well-known cathode ray tube (CRT) is widely used for television (TV) and computer displays. Other display technologies such as the transmissive liquid crystal display (LCD) panel are widely used in certain specialized applications such as displays for portable computers and video projectors.
Market demand is continuously increasing for video displays with higher resolution, greater brightness, lower power, lighter weight, and more compact size. But, as these requirements become more and more stringent, the limitations of conventional CRTs and LCDs become apparent. Microdisplays the size of a silicon chip offer advantages over conventional technologies in resolution, brightness, power, and size. Such microdisplays are often referred to as spatial light modulators (SLMs) since, in many applications, (for example, video projection) they are not viewed directly but instead are used to modulate an incident light beam which forms an image projected on a screen. In other applications such as ultraportable or head-mounted displays, an image on the surface of the SLM may in fact be viewed by the user directly or through magnification optics.
CRTs currently dominate the market for desktop monitors and consumer TVs. But large CRTs are very bulky and expensive. LCD panels are much lighter and thinner than CRTs, but are prohibitively expensive to manufacture in sizes competitive with large CRTs. SLM microdisplays enable cost-effective and compact mid-sized projection displays, reducing the bulk and cost of large desktop monitors and TVs. Desktop computer monitors that would be unreasonably bulky using CRTs and too expensive using LCDs will be cost-effective and compact using SLMs.
Transmissive LCD microdisplays are currently the technology of choice for video projection systems. But, one disadvantage of LCDs is that they require a source of polarized light. LCDs are therefore optically inefficient. Without expensive polarization conversion optics, LCDs are limited to less than 50%-efficient use of an unpolarized light source. Unlike LCDs, micromirror-based SLM displays can use unpolarized light. Using unpolarized light allows projection displays using micromirror SLMs to achieve greater brightness than LCD-based projectors with the same light source, or equivalent brightness with a smaller, lower-power, cheaper light source.
The general operation and architecture of SLMs and SLM-based displays is well known in the industry as shown, for example, in U.S. Pat. No. 6,046,840, U.S. Pat. No. 5,835,256, U.S. Pat. No. 5,311,360, U.S. Pat. No. 4,566,935, and U.S. Pat. No. 4,367,924, the disclosures of which are each hereby incorporated by reference.
FIG. 1 shows the optical design of a typical micromirror SLM-based projection display system. A light source 200 and associated optical system, comprising optical elements 202a, 202b, and 202c, focus a light beam 206 onto the SLM 204. The pixels of the SLM are individually controllable and an image is formed by modulating the incident light beam 206 as desired at each pixel. Micromirror-based projection displays typically modulate the direction of the incident light. For example, to produce a bright pixel in the projected image, the state of the SLM pixel may be set such that the light from that pixel is directed into the projection lens 208. To produce a dark pixel in the projected image, the state of the SLM pixel is set such that the light is directed away from the projection lens 208. Other technologies, such as reflective and transmissive LCDs, use other modulation techniques such as techniques in which the polarization or intensity of the light is modulated.
Modulated light from each SLM pixel passes through a projection lens 208 and is projected on a viewing screen 210, which shows an image composed of bright and dark pixels corresponding to the image data loaded into the SLM 204.
A xe2x80x98field-sequential colorxe2x80x99 (FSC) color display may be generated by temporally interleaving separate images in different colors, typically the additive primaries red, green, and blue. This may be accomplished as described in the prior art using a color filter wheel 212 as shown in FIG. 1. As color wheel 212 rotates rapidly, the color of the projected image cycles rapidly between the desired colors. The image on the SLM is synchronized to the wheel such that the different color fields of the full-color image are displayed in sequence. When the color of the light source is varied rapidly enough, the human eye perceives the sequential color fields as a single full-color image.
Other illumination methods may be used to produce a field-sequential color display. For example, in an ultraportable display, colored LEDs could be used for the light source. Instead of using a color wheel, the LEDs may simply be switched on and off as desired.
An additional color technique is to use more than one SLM, typically one per color, and combine their images optically. This solution is bulkier and more expensive than a single-SLM solution, but allows the highest brightness levels for digital cinema and high-end video projection.
In a CRT or conventional LCD panel the brightness of any pixel is an analog value, continuously variable between light and dark. In fast SLMs, such as those based on micromirrors or ferroelectric LCDs, one can operate the pixels in a digital manner. That is, pixels of these devices are driven to one of two states: fully on (bright) or fully off (dark).
To produce the perception of a grayscale or full-color image using such a digital SLM, it is necessary to rapidly modulate the pixels of the display between on and off states such that the average of their modulated brightness waveforms corresponds to the desired xe2x80x98analogxe2x80x99 brightness for each pixel. This technique is generally referred to as pulse-width modulation (PWM). Above a certain modulation frequency, the human eye and brain integrate a pixel""s rapidly varying brightness (and color, in a field-sequential color display) and perceive a brightness (and color) determined by the pixel""s average illumination over a video frame.
FIG. 2a illustrates a typical display system including an SLM 204 and associated control circuitry 300. A video signal source 301, such as a television tuner, MPEG decoder, video disc player, video tape player, PC graphics card, or the like, provides a video signal 304 in any standard format. If necessary, a conversion circuit 302 performs any necessary conversion operations, such as analog to digital conversion, decompression, or luminance/chrominance decoding, in order to convert the provided video signal into digital RGB pixel data 306.
A display controller 308 accepts the incoming pixel data 306, converts it to bit-plane format, and stores it in a frame buffer 310. Display controller 308 retrieves stored bit-plane-formatted data from the frame buffer and provides it to SLM 204 over a data bus 312 according to a predetermined algorithm, such that each pixel displays data from each bit-plane for a duration proportional to that bit-plane""s desired PWM weighting, thereby producing a grayscale or color image. Addressing and control signals 404 control which SLM pixels are updated with each write operation.
An alternative display system architecture is shown in FIG. 2b. In a standalone application such as in a video-camera or still-camera viewfinder, personal digital assistant (PDA), or a next-generation mobile phone, display controller 308 presents a RAM-like interface 315 to the system""s microprocessor 314. Display controller 308 interleaves the microprocessor""s frame-buffer read and write operations with the steady stream of read operations moving data from the frame buffer 310 to SLM 204. In another implementation, display controller 308 shares the frame buffer 310 with the system""s microprocessor 314 as shown in FIG. 2c. 
Depending on the application, display controller 308, frame buffer 310, and SLM 204 may be separate devices. Alternatively, two or more of these system components may be integrated onto a single chip.
FIG. 3 illustrates the architecture of SLM 204. Incoming data from the data bus 312 is loaded into bitline driver 402 and driven on the bitlines 400 to the array of memory cells 401. It will be apparent to one of ordinary skill in the art that the width of data bus 312 may be made smaller than the number of bitlines 400 using a shift register or similar structure in bitline driver 402 and using multiple clock cycles to load data into bitline driver 402.
Addressing signals 404 control a row decoder 406 to enable a wordline 412, which causes data to be written from bitlines 400 to a row of the memory cells 401 controlling the states of the light modulating elements 410. Each memory cell 401 allows the written pixels 410 to retain their states until next written. In the intervening time, other rows of the display may be updated. The memory cells 401 may be any well-known data storage circuit such as an SRAM, DRAM, or latch. Alternatively, for some types of light modulating elements 410, the xe2x80x98memoryxe2x80x99 may be provided by the inherent bistability of the light-modulating element 410 itself.
A critical constraint on the system design is that the bandwidth or throughput of the SLM data bus 312 is limited. It is possible to increase the throughput of this interface by raising its clock frequency or increasing its bus width. However, these solutions adversely impact the total complexity and cost of the system. Systems that make most efficient use of the available bandwidth between display controller and SLM can use the smallest bus width and/or the lowest bus frequency and will therefore have a cost advantage over less bandwidth-efficient systems.
The prior art in the field of SLMs contains many different methods of controlling an SLM to produce PWM grayscale or color displays. These PWM methods typically share the following goals:
1. Accurately reproduce the desired average signal level and waveform;
2. Maximize optical efficiency by avoiding xe2x80x98dead timesxe2x80x99 when a pixel is always off,
3. Maximize bandwidth efficiency by maximizing temporal regularity of activity on the data bus to the SLM;
4. Minimize perceptual artifacts produced by PWM waveforms; and
5. Achieve the above goals with minimum system complexity and cost.
Improving optical efficiency is desirable since it allows for achieving the same system brightness with a lower-power, smaller, cheaper light source. Improving bandwidth efficiency allows for the use of fewer and/or lower-speed data signals to the SLM, thereby reducing packaging cost and system cost. It is also desirable that the system have the flexibility to implement many alternative PWM waveforms in order to fine-tune the system to minimize visual artifacts due to the use of PWM.
As discussed in U.S. Pat. No. 5,731,802, for example, simultaneously achieving the above goals is difficult. Numerous prior methods have less-than-ideal optical efficiency and bandwidth efficiency. For example, methods such as those described in U.S. Pat. Nos. 5,798,743 and 5,745,193 illustrate the challenge of achieving both optical efficiency and bandwidth efficiency. These methods include significant pixel dead times when light is being wasted, and both are somewhat bandwidth-inefficient due to their non-uniform data throughput over the duration of a video frame.
Attempting to show a single bitplane on the entire display at once works poorly due to the extreme bandwidth demands required. Methods such as those described in U.S. Pat. Nos. 5,619,228, 5,497,172 and 5,731,802, achieve better performance by interleaving data from two, three, or more bitplanes, and, at any one time, displaying the data from several different bit-planes on different areas of the display. In this way, the bandwidth load can be distributed more evenly over the frame period. However, these algorithms are difficult to generalize to arbitrary binary or non-binary PWM weightings and arbitrary array sizes.
Some systems, such as those described in U.S. Pat. Nos. 5,278,652 and 5,731,802, rely on clearing the states of pixels to achieve the desired PWM interval weightings. However, clearing methods add undesired complexity to the design of the SLM array and associated control circuitry, and result in pixel dead times which reduce optical efficiency.
Finally, in prior field-sequential-color systems, such as that described in U.S. Pat. No. 5,448,314, the SLM""s data bus is idle during the blanking intervals between color fields, wasting bandwidth that might otherwise be put to productive use and unnecessarily extending the amount of pixel xe2x80x98dead time.xe2x80x99 In this example of the prior art, after the blanking interval ends, significant dead time elapses before the PWM waveforms for all rows of the display have begun, contributing to additional optical inefficiency.
According to the present invention, methods and apparatus are disclosed for producing a pulse-width-modulated (PWM) grayscale or color image using a binary spatial light modulator. By using novel techniques to stagger and re-quantize the rows"" PWM intervals to a clock of a period based on the frame time divided by number of rows in the display, the system""s peak bandwidth requirements are optimized for displays of arbitrary resolution and arbitrary choice of PWM waveform. Additionally, use of a gating circuit increases the optical efficiency of a spatial light modulator using these PWM techniques in a field-sequential color system by reducing the duration of the blanking period between color fields to the minimum allowed by the data bus bandwidth of the SLM. The gating circuit of the present invention allows an SLM to be preloaded with data during the blanking interval and eliminates pixel dead time after the end of the blanking interval. Optical efficiency and bandwidth efficiency are therefore improved.
The techniques of the present invention provide a grayscale display of arbitrary resolution capable of displaying arbitrary PWM waveforms, which achieves up to 100% bandwidth efficiency, and up to 100% optical efficiency. Such grayscale performance can be achieved using a simple passive, SRAM, DRAM, or latch-based SLM architecture without the complexity and cost of additional SLM circuitry for clearing or double-buffering.
The techniques of the present invention also provide a field-sequential color display of arbitrary resolution capable of displaying arbitrary PWM waveforms, which achieves up to 100% bandwidth efficiency, and improved optical efficiency over the prior art. In particular, pixel xe2x80x98dead timexe2x80x99 is minimized when switching between color fields. A gating circuit allows inter-field dead time to be reduced to a duration limited only by the bandwidth of the SLM interface and the rate at which the illumination system can change the color of the light illuminating the SLM.
Such optical efficiency for field-sequential color is achieved using a simple SRAM or DRAM-based SLM architecture or the like, without the complexity and cost of double-buffering or multiple bits per pixel, when used in conjunction with a simple gating circuit of the system as disclosed herein. For some types of SLMs, such as electrostatically actuated micromirrors, implementation of the gating circuit allows the system to temporarily disable the bias voltage to the light-modulating elements or to temporarily disable illumination of the light-modulating elements, and no additional blanking circuitry within the SLM itself is necessary.
According to an aspect of the present invention, a method is provided for driving a spatial light modulator (SLM), wherein the SLM has a plurality of rows, each row having a plurality of pixels, each pixel comprising a storage bit and a light-modulating element, wherein each of the plurality of rows is updated one or more times during each of a plurality of frames to be displayed by the SLM. The method typically comprises the steps of, during each frame, selecting the rows of the SLM in an update sequence having a plurality of update events, wherein each update event in the update sequence corresponds to a predetermined row of an image and one of a plurality of predetermined bitplanes of the image, each bitplane having a predetermined pixel waveform segment duration; providing a plurality of image data signals to the SLM at each update event, such that the selected row of the SLM is updated with image data corresponding to the selected row and bitplane of the image; and staggering, by a stagger interval, the update events of each row relative to the corresponding update events of a previous row in a row order, wherein during each stagger interval a number of update events occurs, the number of update events occurring in the SLM during each stagger interval being equal to the number of update events occurring for each row during a frame.
According to another aspect of the present invention, a spatial light modulator (SLM) is provided. The SLM typically comprises an array of pixel elements, an array of memory cells coupled to the array of pixel elements and having a plurality of rows, wherein each memory cell controls the state of one of the pixel elements. The SLM also typically includes a plurality of bitlines for providing data signals to the array of memory cells, one row at a time, and a row decoder, wherein the row decoder selects, in response to a row address, one of the plurality of rows of memory cells such that the selected row of memory cells is updated with the data signals provided on the bitlines. In typical operation, during each frame, the rows of the SLM are updated in an update sequence comprising a plurality of update events, each update event in the update sequence corresponding to a predetermined row of an image and one of a plurality of predetermined bitplanes of the image, each bitplane having a predetermined pixel waveform segment duration, and the update events of each row are staggered, by a stagger interval, relative to the corresponding update events of a previous row in a row order, wherein during each stagger interval a number of update events occurs, the number of update events occurring in the SLM during each stagger interval being equal to the number of update events occurring for each row during a frame.
According to yet another aspect of the present invention, a spatial light modulator (SLM) is provided. The SLM typically comprises an array of pixel elements and an array of memory cells coupled to the array of pixel elements and having a plurality of rows, wherein each memory cell controls the state of one of the pixel elements. The SLM also typically includes a blanking means, coupled to the pixel elements, for simultaneously forcing all pixel elements to an off state in response to a blanking signal. The blanking means may include any one of the following:
any of a plurality of logical gating circuits such as a AND, OR, NAND and NOR gate;
a switching circuit for disabling a pixel bias voltage; and
a circuit for disabling illumination of the pixel elements.
According to a further aspect of the present invention, a spatial light modulator (SLM) is provided. The SLM typically comprises an array of pixel elements and an array of memory cells coupled to the array of pixel elements and having a plurality of rows, wherein each memory cell controls the state of one of the pixel elements. The SLM also typically includes a plurality of gating circuits, each gating circuit coupled to one of the pixel elements. In typical operation, when a blanking control signal is applied to the gating circuits, all associated pixel elements are simultaneously forced to an off state regardless of the content of the associated memory cells.
According to still a further aspect of the present invention, a spatial light modulator (SLM) is provided. The SLM typically comprises an array of pixel elements and an array of memory cells coupled to the array of pixel elements and having a plurality of rows, wherein each memory cell controls the state of one of the pixel elements. The SLM also typically includes a switching circuit coupled to all of the pixel elements for providing a bias voltage to all the pixel elements. In typical operation, when the bias voltage is at a first level the state of each pixel is controlled by the control voltage from the respective memory cell, and wherein when the bias voltage is at a second level all pixel elements are in an off state, and when a blanking signal is applied to the switching circuit, the switching circuit switches the bias voltage to the second level such that all pixel elements are simultaneously forced to an off state regardless of the applied control voltages.
According to yet a further aspect of the present invention, a method is provided for driving the pixels of a spatial light modulator (SLM) in a field-sequential color (FSC) display system. The SLM typically includes an array of memory cells coupled to an array of pixel elements, the array of memory cells comprising a plurality of rows, wherein each memory cell controls the state of one of the pixel elements, wherein the FSC system includes a color generating mechanism capable of illuminating the pixel elements with multiple color fields. The method typically comprises the steps of illuminating the pixel elements with the multiple color fields in a cyclical manner, wherein each color field illuminates the SLM one or more times during a frame, and, during each field, selecting the rows of the SLM in an update sequence having a plurality of update events, each update event in the update sequence corresponding to a predetermined row of an image and one of a plurality of predetermined bitplanes of the image, each bitplane having a predetermined pixel waveform segment duration, and providing a plurality of image data signals to the SLM at each update event, such that the selected row of the SLM is updated with image data corresponding to the selected row and bitplane of the image. The method also typically includes the steps of, between each subsequent color field, blanking all pixel elements for an interval having a predetermined duration, and during each blanking interval, pre-loading the memory cells of the SLM such that when the blanking interval ends, the next color field""s update sequence may be resumed in a continuous manner so as to eliminate pixel dead time after the end of the blanking interval.
According to an additional aspect of the present invention, a method is provided for reducing an amount of color breakup perceived by a viewer in a field-sequential color (FSC) system having a spatial light modulator (SLM) driven by bitplane data signals, wherein the SLM includes an array of memory cells coupled to an array of pixel elements, wherein each memory cell controls the state of one of the pixel elements, wherein the FSC system includes a color generating mechanism capable of illuminating the pixel elements with multiple color fields. The method typically comprises the steps of illuminating the pixel elements with the multiple color fields in a cyclical manner, wherein each color field illuminates the SLM during each cycle, providing bitplane data signals to the memory cells such that during each color field each of a plurality of rows of memory cells is updated by one or more of a plurality of update bitplanes, each update bitplane having a predetermined weight, and simultaneously blanking all pixel elements one or more times during each separate color field for an interval having a predetermined duration, so as to split each color field into two or more subfields. The method also typically comprises the steps of simultaneously blanking all pixel elements between each separate color field for the interval having the predetermined duration, and during each blanking interval, preloading the memory cells with data such that when the blanking interval ends, the update sequence may be resumed in a continuous manner for the next color field or subfield.
According to yet an additional aspect of the present invention, a method is provided for driving a spatial light modulator (SLM), wherein the SLM has a plurality of rows, each row having a plurality of pixels, wherein each pixel includes a storage bit and a light-modulating element, and wherein each of the plurality of rows is updated with pixel data at each of a plurality of update events during each of a plurality of frames to be displayed by the SLM, wherein each update event has a predetermined weight. The method typically comprises the steps of, for each frame, writing pixel data associated with a first bitplane and a first one of the plurality of rows to the first row at a first update time, and writing pixel data associated with the first bitplane and a second one of the plurality of rows to the second row at a second update time different from the first update time by a stagger interval with duration equal to the frame duration divided by the number of the plurality of rows.
According to yet an additional aspect of the present invention, a method is provided for driving a spatial light modulator (SLM), wherein the SLN has a plurality of rows, each row having a plurality of pixels, wherein each pixel includes a storage bit and a light-modulating element, and wherein each of the plurality of rows is updated with pixel data at a plurality of update events, the events corresponding to at least two bitplanes, during each of a plurality of frames to be displayed by the SLM, wherein each update event has a predetermined weight. The method typically comprises the steps of, for each frame, for each row, writing to the row pixel data associated with the row and a first bitplane at a first update event, the first update event occurring at a first update time wherein the first update time for the row is staggered from the first update time of the previous row by a stagger interval with duration equal to the frame duration divided by the number of the plurality of rows, and for each row, writing to the row pixel data associated with the row and a second bitplane at a second update event, the second update event occurring at a second update time, wherein the second update time for the row is different from the first update time for the row by a duration based on the weight corresponding to the first update event, and wherein the second update time for the row is different from the second update time of the previous row by the stagger interval.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.